Source of ionizing beam



M. L. VESTAL SOURCE OF' IONIZING BEAM Filed June 5, 1962 IIIIIIIII HQE NEGATIVE ION DETECTOR POSITIVE ION DETECTOR Sept. 29, i964 ifi/ifi IONIZATION REGION INVENTOR.

MARV/P/ L. VESTAL ATTORNEYAB E.G.ELECTRON SOURCE FIELD FREE BY hw, M, JM@ fm/f;

III-

#MIMI oR OTHER souRcE United States Patent tion of Maryland Filed .lune 5, 1962, Ser. No. 209,1l84 8 Claims. (Cl. 31E- 15) This invention relates to the production of ionizing beams, and more particularly, to formation of an ionizing beam of extremely restricted size in one direction, though of large extent in two mutually perpendicular directions.

The formation of ionizing beams is very useful in se eral diiierent elds of scientific endeavor. One particular use for an ionizing beam is in mass spectrometry, in which the material under study is subjected to an ionizing beam and the resultant ionization products may be examined to determine, for instance, the nature of the material. A relatively new technique for such examination is that employing a coincidence phenomenon, as more fully disclosed in the patent to Henry M. Rosenstock, No. 2,999,157, issued September 5, 1961, and assigned to the assignee of the present invention. In contrast with earlier types of mass spectrometry, the coincidence technique involves detection of both the negative and positive ionization products resultant from exposure of the material under investigation to the ionizing beam. As illustrated in the drawing of the Rosenstock patent, the beam is directed perpendicularly with respect to an electric eld which urges the negative and positive ionization products toward separate detectors.

It will be evident from a consideration of the geometry of this apparatus that it is desirable that the beam cover a relatively large cross-sectional area perpendicular to the electric field operative upon the ionization products. However, it will also be evident that it is undesirable that the beam be of substantial width parallel to the direction of the electric field, for if it were of such substantial width, the beam would cause ionization events at different distances from the detectors and the resultant products would necessarily arrive at Athe detectors at different times by reason both of the diiierent spacings between the detectors and the ionized particles at the moment of ionization, and the diiierent field lintensities at such different locations.

It is a primary object of this invention to restrict the ionizing beam to an extremely small width parallel to the electric field of the Rosenstock apparatus, but yet to form a beam which is of relatively great extent in both of the perpendicular directions.

While it is known, particularly in the cathode ray lield, to form a beam of charged particles which is of extremely restricted dimension in one direction, conventional tech-- niques for formation of such a beam necessitate that the beam be also of very restricted extent in at least one of the perpendicular directions. One illustration of such known techniques is in the formation of an extremely small spot for use in the scanning of a fluorescent target in a cathode ray oscilloscope. In contrast, the apparatus of this invention is not designed to form a small spot but rather to form a nearly planar beam.

The above recited and other objects of the invention are accomplished by aligning a source of an ionizing beam with a plurality of metal plates, each of which has means thereon defining a slit of extremely small dimension in one direction. Such means desirably consist of metal coverings on the metal plates of thicknesswhich are so small that the cross-sectional area of each slit is substantially greater than the surface area of the metal covering defining the slit. Two such plates are spaced apart by a distance which is very much greater than the thickness of each slit, with the two plates preferably di- 3,151,269 Patented Sept. 29, 196,4

ICC

rectly connected together electrically so as to deiine a ield-free region between the plates. The source of the ionizing beam is aligned with the slits. The apparatus of the invention further includes at least a third guard plate having means thereon deiining a slit of somewhat greater extent in said one direction than the slits in the other two plates. A guard plate is positioned on the far side of at least the second of the other two plates, and v preferably also the first plate, and a retarding electric field is formed between the guard plate and its adjacent plate to urge secondary electrons back toward the adjacent plates.

While the beain may be composed of photons, the invention has its greatest advantage in connection with the formation of a beam of charged particles, such as electrons, protons, or alpha rays. Such particles may be urged through the slits in the metal plates by an electric iield formed between the first plate and another plate positioned adjacent the source and having a slit therethrough of much greater size than the slits in the other plates.

It will be evident that, though the invention is particularly designed for use in connection withV the coincidence mass spectrometer described in the Rosenstock patent, it is not limited to such use. Actuallythe invention may be used wherever it -is desired to form an ionizing beam, either of charged particles, or of photons, which is of extremely narrow dimension in one direction but of much greater dimensions in each of the two perpendicular directions.

The invention will now be more fully described in conjunction with drawings showing a preferred embodiment thereof.

In the drawings:

FIG. l is a schematic illustration of a portion of the coincidencemass spectrometer of the Rosenstock patent, with a schematic illustration of the plates of the present invention;

FIG. 2 is a schematic illustration of the alignment and connections of the several plates of the invention; and

FIG. 3 is a vertical elevational view of one of the plates of the invention, taken generally along the line 3 3 of FIG. VAl.

The coincidence mass spectrometer of the Rosenstock patent includes a container 1t) within which ionization events are to be caused by an ionizing beam emanating from a source 11. The container may appropriately be evacuated butV supplied with a gaseous sample. of the material to be investigated, as will be evident from the Rosenstock patent. The container also has positioned therein means (not shown) creating an electric ield'fE of direction indicated by the arrow in FIG. 1, this field being designed to urge positive ions in the direction of the ield toward a positive ion detector 12, and negative ions in the opposite direction toward a negative ion detector 13.

' The ionizing beam from the source 11 passes through a window 14 in the container which is transparent tothe beam and causes ionizing events along the shaded path 15, this path being of a width in the direction parallel to the field E determined by the width of the ionizing beam. As explained above, it is extremely important that the width of this beam be as small as possible, and the purpose of the present invention is to provide a beam of these characteristics. It is also important, however, that the beam be of great extent in directionsperpendicullar to the beam E both in the plane of the drawing and a plane perpendicular thereto, so that as many ions as possible may be formed for study. This beam is formed i in such coniguration by an arrangement of electrodes generally shown in FIG. l at 156.

Referring now to FIG. 2, showing the'electrodes or.

plates or plates le 1n more detail, the photons or charged particles from the source 1l pass through a relatively large slit 17 formed in a metal plate l. They then impinge upon a slit of very much smaller size in a metal plate I9. Actually, this plate and slit are preferably formed by a metal plate of substantial thickness in the direction of the beam having internal walls deiining a passage 2i? therethrough. The size of this passage is not of importance except that it must be very much larger than the size of the slit 21 formed by the internal walls of the metal covering 22 on the plate 19.

The construction of the slip 21 will be evident by consideration of FIG. 3 along with FIG. 2. Particularly by reference to FIG. 3 it will be seen that the slit is extremely narrow in one direction, which is the direction corresponding to that of the electric field E in FIG. l, and that corresponding to the dimension labeled 23 in FlG. 2. The plate and slit may appropriately be formed by plating onto a plate of one metal an extremely small thickness of a different metal, masking all areas of the metal covering except that corresponding to the slit 2l, and then etching through the metal covering by methods Well known in the engraving and printed circuit arts. For instance, the metal plate may be of brass of thickness of the order of 5 mils, while the covering may be of nickel of thickness of the order or 1/10 of one mil. The covering and plate are desirably of different metals for selective etching during the step of forming the slit, but it is not at all essential that this method be used to form the plate and slit and it is not necessary that the two metals be of different chemical composition if some entirely different method for forming the slit is employed. Further, it is not necessary that either or" the two particular metals identified above be used, and the thickness of the plate 19 is not at all important to the operation of the invention. It is necessary, however, that the passage 2i) through the plate i9 be very much larger than the slit 21, and that the thickness of the metal covering 22 be extremely small.

In one embodiment of the invention, the thickness 23 of the slit 2l was of the order of l mil, While the other dimension of the rectangular slit was many times larger. The result of this construction is that the surface area of the walls of `the metal covering defining the slit 21 is very much less than the cross-sectional area of the slit, a condition which is of extreme importance to satisfactory operation of the invention.

The ionizing beam from the source ll is urged through the slits I7 and 21 by an electric field El if the beam is of charged particles. The apparatus shown in the drawings is particularly designed for use with electron beams and a suitable source of voltage such as illustrated by the battery 25 is connected between the plates 18 and 19 with the positive terminal connected to plate 19 and grounded.

At the opposite side of plate is positioned another metal plate 26 which also has inner walls dening a passage 27 therethrough. The passage 27 is desirably larger than the passage 2h, as shown in FG. 2, for reasons which will be shortly explained.

The plate 26 carries a metal covering 23 of characteistics similar to those of metal covering 22, and the covering 23 has internal walls defining a slit Z9 extending therethrough. This slit is desirably substantially larger than the slit 21, for reasons that will be explained.

A further metal plate Sti is positioned parallel to the plate 26 but at a distance therefrom which is much greater than the spacing between plates i9 and 26. The plate 36, like plate 19, has a metal covering 3l which extends over a portion of the passage 32 formed by inner walls of the plate 30. The covering 3i itself has inner walls deiining a slit 33 which is of the order of the same size as the slit 21 in plate 19. Though it is not essential that these two slits be of identical size, they should be about the lsaine size for satisfactory operation of the invention.

is from the plate is r The plates Sti and 26 are preferably directly connected together electrically, so that the beam passes through a field-free region in its passage between slits 29 and 33.

To the opposite side of plate 30 from plate 2.6 there is located a further metallic plate 35 having internal walls defining a passage 36 therethrough. The plate 35 also preferably has a metallic coating 37 thereon which itself has internal walls dening a slit 38 therethrough. Plate 35 and covering 37, as well as the slit 38, may appropriately be of the same characteristics and dimension as the plate 26, the covering 2&5 and the slit 29, respectively.

Assuming that the ionizing source 11 is a source of electrons, the beam of electrons from the source will be collimated to some extent by passage through the slit i7 in the first plate I8. The electric field El will urge the electrons toward the slit 2i, but this beam will be further restricted in its passage through the slit 21 by the much smaller size of that slit than the slit 17. Those electrons which strike covering 22 of course will not be of any further concern in the operation of the apparatus, but there will be some secondary electrons created by impingement of the ionizing beam on the inner walls of covering 22 deiining the slit 21. These secondary electrons of course Will be of much smaller energy than the electrons in the main ionizing beam, so that it is possible to deect them back to the plate 19 by formation of a retarding field E2 created by connection of an electric source such as the battery 40 between the plates 19 and 26. It is not absolutely essential that the plate 26 be used for this guard purpose, since there is a very long field-free region between the plate 19 and the plate Sti, but it is very desirable that no secondary electrons pass through the slit 3S. The guard plate 35 is provided for this purpose and has connected between it and the plate 3i) a voltage source such as battery 41 to form a retarding electric field E3 for secondary electrons created bly impingement of the electron beam on the edges of s it 33.

The voltages of batteries 40 and 41 may of course be very much less than that of battery 25. A suitable set of values might be about G volts for the latter and 50 volts for the former.

As indicated, the slits 29 and 3S are of substantially greater extent parallel to the dimension 23 than is either of slits 2]. and 33. This is for the reason that it is desirable that no undeviated electrons strike the edges of either of slits 29 and 38, since any such electrons would not necessarily follow the very narrow path desired.

The operation of the apparatus of the invention will be apparent from the description hereinabove. It will be evident that the invention makes it possible to form a collimated beam which is of extremely small dimension in one direction without the accompaniment of secondary electrons which would tend to interfere with an ionizing process. To further discourage secondary electrons from accompanying the main beam, it would be possible and in some cases desirable, to add additional combinations similar to plates 30 and 35.

Certain alternative characteristics and constructions from those specically illustrated in the drawings have been suggested herein. It will be evident that many other minor changes in the illustrated embodiment can be made without departure from the scope of the invention. The invention is therefore not to be considered limited to the illustrated embodiment, but rather only by the scope of the appended claims.

I claim:

l. Apparatus for supplying an ionizing beam of extremely narrow dimension in one direction comprising a first and a second spaced but substantially parallel plates, each having thereon means deiining a slit of extremely narrow dimension in said one direction and of very small thickness such that the surface area of the Walls of said slit is much smaller than the cross-sectional area of the slit,

means including an ionizing source positioned at the side of said first plate opposite said second plate for directing an ionizing beam through said slits,

and a third plate positioned at the side of said second plate remote from said first plate and substantially parallel thereto, said third plate having means thereon defining a slit aligned with the slits in said first and second plates but larger' than the slit in said second plate, and means connected to said second and third plates for forming an electric field therebetween in the direction of the second plate to urge secondary electrons created by impingement of said beam onto said slit-defining means from the region between said second and third plates back toward the second late. 2. 1r)The apparatus of claim l in which the spacing between said first and second plates is much greater than the spacing between said second and third plates and said first and second plates are at substantially the same electrical potential to form a substantially electric-fieldfree region therebetween.

3. The apparatus of claim 2 further including a fourth plate positioned between said first and second plates and parallel thereto but located much closer to the first than to the second plate, said fourth plate having thereon means defining a slit aligned with but larger than the slit in said first plate,

and means connected to said rst and fourth plates for forming an electric field therebetween in the direction of said first plate to urge secondary electrons created by impingement of said beam onto said first plate back toward the first plate.

4. The apparatus of claim 3 in which said ionizing source supplies a beam of charged particles,

and including means connected to said first plate operable to form an electric field between the source and the first plate in a direction such as to urge the particle-s along the path through said slits.

5. The apparatus of claim 4 in which said last-named means includes, i

a fifth plate positioned between said source and said first plate and substantially parallel to the first plate, said fifth plate having means thereon defining a slit therethrough much larger than the slit in said first plate but aligned therewith,

and a source of voltage connected between said fifth and first plates.

6. Apparatus for supplying a beam of charged particles of extremely narrow dimension in one direction for use in ionizing a material over a relatively large area but only a very small thickness perpendicular to the beam path, comprising three parallel spaced metal plates, the first and second of which are close together in comparison with the distance between the second and third plates, each plate having internal walls defining a passage therethrough,

the second and third plates each having a metal covering on one surface of the plates of a Very small thickness which is very much less than the plate thickness, said surfaces being parallel to each other,

each metal covering having walls extending over a portion of the passage through its plate but defining a slit through the covering of extremely narrow dimension in said one direction, the cross-sectional area of each slit being very much greater than the surface area of the walls defining the slit,

a source of charged particles aligned with said slits and passages,

means connected to said first and second plates for forming an electric field therebetween in a direction such as to urge the charged particles through the slit in said second plate and toward the slit in said third plate, said second and third plates being at substantially the same electrical potential so as to p create a substantially electric-field-free region therebetween,

a fourth metal plate parallel to said third plate and spaced rather closelythereto on the side opposite said second plate, said fourth plate having Walls de-l fining a passage therethrough corresponding with and aligned with the passages in said other plates, and having a metal covering thereon of thickness corresponding to those of said other coverings, said covering having walls defining a slit aligned with said other slits but of greater width in said one and the perpendicular direction,

and means connected to said third and fourth plates for forming an electric field therebetween in the direction of the third plate for urging secondary electrons created by impingement of said beam onto the walls defining the slit in said third plate back toward the third plate from the region between said third and fourth plates.

7. The apparatus of claim 6 including a fifth metal plate parallel to said second plate, positioned between it and said third plate but much closer to the second than the third plate, said fifth plate having walls defining a passage therethrough corresponding with and aligned with the passages through said other plates and having a metal covering thereon of thickness corresponding to those of said other coverings, said metal covering having walls defining a -slit aligned with said other slits but of substantially greater width both. in said one direction and the perpendicular direction than the slit in said second plate, and means connected to said second and fifth plates for forming an electric field therebetween in the direction of the second plate for urging secondary electrons created by impingement of said beam onto the Walls defining the slit in lsaid second plate back toward the second plate from the region between said second and fifth plates, said fifth and third plates being electrically connected so as to create an electric field-free region therebetween.

8. The apparatus of claim 7 in which the first mentioned-electric field is of much greater intensity than said second and third mentioned fields.

No references cited. 

1. APPARATUS FOR SUPPLYING AN IONIZING BEAM OF EXTREMELY NARROW DIMENSION IN ONE DIRECTION COMPRISING A FIRST AND A SECOND SPACED BUT SUBSTANTIALLY PARALLEL PLATES, EACH HAVING THEREON MEANS DEFINING A SLIT OF EXTREMELY NARROW DIMENSION IN SAID ONE DIRECTION AND OF VERY SMALL THICKNESS SUCH THAT THE SURFACE AREA OF THE WALLS OF SAID SLIT IS MUCH SMALLER THAN THE CROSS-SECTIONAL AREA OF THE SLIT, MEANS INCLUDING AN IONIZING SOURCE POSITIONED AT THE SIDE OF SAID FIRST PLATE OPPOSITE SAID SECOND PLATE FOR DIRECTING AN IONIZING BEAM THROUGH SAID SLITS, AND A THIRD PLATE POSITIONED AT THE SIDE OF SAID SECOND PLATE REMOTE FROM SAID FIRST PLATE AND SUBSTANTIALLY PARALLEL THERETO, SAID THIRD PLATE HAVING MEANS THEREON DEFINING A SLIT ALIGNED WITH THE SLITS IN SAID FIRST AND SECOND PLATES BUT LARGER THAN THE SLIT IN SAID SECOND PLATE, AND MEANS CONNECTED TO SAID SECOND AND THIRD PLATES FOR FORMING AN ELECTRIC FIELD THEREBETWEEN IN THE DIRECTION OF THE SECOND PLATE TO URGE SECONDARY ELECTRONS CREATED BY IMPINGEMENT OF SAID BEAM ONTO SAID SLIT-DEFINING MEANS FROM THE REGION BETWEEN SAID SECOND AND THIRD PLATES BACK TOWARD THE SECOND PLATE. 